fix broken links in README.md

seems like `building-spark.html` was renamed to `building-with-maven.html`?

Is Maven the blessed build tool these days, or SBT? I couldn't find a building-with-sbt page so I went with the Maven one here.

Author: Ryan Williams <ryan.blake.williams@gmail.com>

Closes #2859 from ryan-williams/broken-links-readme and squashes the following commits:

7692253 [Ryan Williams] fix broken links in README.md